The present invention relates to resinous compositions useful as dielectrics and more particularly to polyphenylene ether-polyepoxide compositions suitable for fabrication into printed circuit boards.
A number of polyphenylene ether-polyepoxide compositions have favorable dielectric properties, and supposedly being useful in circuit board manufacture, are known. However, for the most part these have not attained wide commercial use because of deficiencies in one or more properties. Thus, while the polyphenylene ethers are excellent dielectrics and the properties of combinations thereof with polyepoxides are favorable in this respect, they lack solvent resistance which is required in order for the circuit board to survive cleaning. Other deficiencies are found in areas such as flammability, solderability, and resistance to high temperatures.
In addition to excellent dielectric properties, resinous compositions to be used for printed circuit board manufacture should be highly flame-retardant. A V-1 rating, as determined by Underwriters Laboratories test procedure UL-94, is universally required with V-0 usually being necessary. The V-0 rating requires a flame-out time (FOT) of not more than 10 seconds in any trial and a cumulative FOT of not more than 50 seconds for five samples. As a practical matter, a maximum cumulative FOT of 35 seconds often is mandated by purchasers.
The fabricated board should not lose substantial weight and its surface should not be appreciably marred by contact with methylene chloride, a solvent commonly used for cleaning. Since conductive connections with the printed circuit typically are made by soldering, the board must be solder-resistant as evidenced by the lowest possible percent increase in thickness (Z-axis expansion) when exposed to liquid solder at 288.degree. C. In addition to all these properties of the cured material, a relatively short curing time is highly desirable.
In Japanese Kokai 58/69052, combinations of polyphenylene ethers with various types of polyepoxides are disclosed. The latter include epoxy novolac resins and polyglycidyl ethers of such compounds as bisphenol A and 2,2-bis(3,5-dibromo-4-hydroxyphenyl) propane, commonly referred to as tetrabromobisphenol A. Curing of these compositions is achieved by contact with various known curing agents, including amines. The cured compositions, however, have been found to be seriously deficient in solvent resistance and, in certain cases, in solderability.
Copending, commonly-assigned application Ser. No. 288,214, filed Dec. 22, 1988, discloses curable polyphenylene ether-polyepoxide compositions where the polyepoxide composition is based on a monomeric bisphenol polyglycidyl ether having an average of at most one aliphatic hydroxyl group per molecule and containing about 10%-30% bromine as aryl substituents. This monomeric system exhibits improved processability in the treating operation, but does not yet have cure speed as fast as some manufacturers desire. The high content of brominated monomeric polyepoxide contributes to the cost of such composition also. Finally, Z-direction thermal expansion coefficients of laminates based on this monomeric system may be higher than is acceptable in some uses of laminates made therewith. Accordingly, room for improvement with respect to these factors exists.
In addition, curable compositions suitable for bonding sheet fabrication are needed. Bonding sheets are employed when a multi-layer structure is desired, involving etching of numerous printed circuits followed by their lamination into a single unit. For this purpose, a fiber-reinforced resinous bonding sheet is employed to separate the etched copper circuity on two successive circuit boards, with the desired connections being made through the bonding sheet.
The bonding sheet composition must generally have a substantially higher flow rate when melted under low pressure than a composition employed in circuit board manufacture. It must also have a relatively high resin loading, since the resin must completely fill the voids created during the etching of circuits in the printed circuit boards. Extended cure time also is necessary, in order that the required flow may be achieved before curing time is initiated. The formulation must be compatible with the base material in the circuit board.